Fluid systems, such as cooling, heating, petroleum refining, pneumatic or other vapor or gas system, waste water control, or chemical process systems, typically utilize valves to control or otherwise regulate fluid flow. These fluid control devices may include a variety of different types, sizes, and configurations of valves, such as globe valves, ball valves, butterfly valves, and plug valves. Several factors affect the design or choice of fluid control valves for any particular fluid application. As used herein fluid may encompass, liquid, vapor, gas, or a mixture of any of these fluid phases. For example, design consideration such as noise, pressure, and temperature, to name a few, may influence the type, size, and construction of any particular fluid control device chosen for a particular application.
Particular flow control valves typically include a body having an inlet and an outlet and a valve element between. In a ball valve, for example, the inlet, outlet, and valve element have a bore for allowing flow through the valve. At a high flow rate through the valve, the bore in the valve element may be fully aligned with the bores in the inlet and outlet of the valve body. However, at a low flow rate, the bore in the valve element may be substantially out of alignment with the bores in the valve inlet and valve outlet so as to restrict flow through the valve. At intermediate flow rates the bore in the valve element may be aligned in an intermediate position. When the bore in the valve element is out of alignment with the bores in the inlet and outlet of the valve body, the valve is in a throttling position restricting flow through the valve, which may introduce a loss of fluid pressure to the fluid being throttled.
Localized high-pressure drops may cause vibration and noise problems. For example, one problem associated with the throttling of pipeline flows with valves is the occurrence of “noise,” which is caused by vibration in the valve and pipe caused by valve induced shear turbulence and vortices formed in the fluid passing through the valve.
Additionally, a portion of the valve element body may extend into the flow path at the valve inlet and present a reduced cross-section flow path for the fluid flowing through the valve. This reduced area may cause the fluid to accelerate into the valve element while the fluid stream loses pressure, i.e., the fluid expands as it enters the valve element. These changes may be non-uniform in the flow path through the valve, because portions of the fluid flow may not be directly affected by the movement of the ball to alter the flow path through the ball, whereas other portions of the fluid stream may be substantially effected. As a result, shear occurs within the fluid stream where high pressure, low velocity portions of the fluid stream contact high velocity, low pressure fluid stream areas through the valve. This shear can induce noise and/or cavitation in the valve which can destroy the valve or render it unsuitable for use in noise sensitive areas.
For cavitation or noise attenuation, it may be desirable to spread the pressure drop taken by a valve over as many components within the valve as possible. Each component has a pressure drop ratio: a ratio of pressure drop divided by inlet pressure to that component. Under specified conditions, a valve restricts flow which results in an overall pressure drop. This overall pressure drop may be divided among the various components within the valve that contribute to flow restriction and thus to the overall pressure drop.